CREATE TRIGGER name { BEFORE | AFTER } { event [OR ...] }
ON table FOR EACH { ROW | STATEMENT }
EXECUTE PROCEDURE func ( arguments )
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CREATE TRIGGER will enter a new trigger into the current data base. The trigger will be associated with the relation table and will execute the specified function func. Only the relation owner may create a trigger on this relation.
The trigger can be specified to fire either before BEFORE the operation is attempted on a tuple (before constraints are checked and the INSERT, UPDATE or DELETE is attempted) or AFTER the operation has been attempted (e.g., after constraints are checked and the INSERT, UPDATE or DELETE has completed). If the trigger fires before the event, the trigger may skip the operation for the current tuple, or change the tuple being inserted (for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the last insertion, update, or deletion, are "visible" to the trigger.
Check if the specified distributor code exists in the distributors table before appending or updating a row in the table films:
CREATE TRIGGER if_dist_exists
BEFORE INSERT OR UPDATE ON films FOR EACH ROW
EXECUTE PROCEDURE check_primary_key ('did', 'distributors',
'did');
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Before canceling a distributor or updating its code, remove every reference to the table films:
CREATE TRIGGER if_film_exists
BEFORE DELETE OR UPDATE ON distributors FOR EACH ROW
EXECUTE PROCEDURE check_foreign_key (1, 'CASCADE',
'did', 'films', 'did');
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